1. Field of the Invention
The present invention relates to electromagnetic wave absorbing elements arranged on interior surfaces of a housing to constitute an electromagnetically dark room.
2. Prior Art
It is well known that a plurality of electroconductive film elements are arranged in parallel crosses on a wall so as to absorb electromagnetic wave energy incident normal to the wall to thereby suppress the transmittance and reflection of the incident electromagnetic wave. This conventional technology is disclosed in, for example, the Electrical Communication Society Report, Vol. 50, No. 3 (March 1967), pp 416-423. Namely, the electrical field vector E of an incident electromagnetic wave is needed to align in parallel to the electroconductive films in order to be absorbed. In the conventional structure, the electroconductive film elements are arranged in parallel crosses so that all of the horizontal and vertical vector components of the electromagnetic wave can align in parallel to the electroconductive films.
Further, recently it is theoretically suggested that an absorbing body having a varying absolute value .vertline..epsilon..vertline. of complex permittivity varying gradually along the advancing direction of an incident electromagnetic wave can absorb the electromagnetic wave to suppress the reflection thereof. However, it is difficult to arrange the permittivity .epsilon. to gradually change. Therefore, it is practically proposed to stack a plurality of absorbing body segments having different permittivities on a wall so as to change the permittivity discretely and step wisely along the incident wave.
Further, it is known that a plurality of pyramid bodies each composed of plastic foam containing a considerable amount of electroconductive carbon black are arranged such that the top vertexes of the pyramids are directed toward the incident electromagnetic wave so that the surface electric resistivity of the bodies can be gradually changed as a whole structure along the incident wave direction.
In the second-mentioned conventional structure in which absorbing body segments of different permittivities are stacked on a wall, the permittivity cannot be gradually changed, thereby failing to effectively suppress the reflection of the incident wave. Moreover, such structure has practical problems that the stacked body segments have a considerably heavy total weight, and manufacturing thereof requires a considerable burden.
In the third-mentioned conventional structure comprised of pyramid plastic-mold bodies containing carbon black, a great content of carbon black is needed to increase the absorption efficiency, resulting in increase of the body weight. Further, due to the moisture absorbing capacity of the carbon black, air conditioning equipment is needed to avoid the deterioration of electromagnetic wave absorption property.